1. Technical Field
The present invention relates to a diagnostic method for an engine and, more particularly, to an algorithm for determining the top dead center location in an internal combustion engine.
2. Discussion
As is commonly known in the field of internal combustion engine design, it is advantageous to know the relative position of each piston within its combustion cylinder in order to properly time an ignition signal or a fuel injection signal or both. Specifically, it is beneficial to know the top dead center (TDC) position of a piston because it is at this time where compression is at its peak. Unfortunately, once the engine has been assembled, the piston is concealed from view and determination of the top dead center is difficult.
In the past, a variety of methods and apparatus have been used for determining the piston top dead center position. One such system involves marking a rotating component coupled to the crankshaft such as a harmonic balancer or an engine flywheel relative to a sensor or scale located at a fixed point on the engine. Theoretically, the marking aligns with the fixed reference point at TDC. Unfortunately, this system suffers from the inaccuracies involved with mechanically positioning the markings or sensors to correspond to the actual position of the piston within the cylinder. Regardless of the amount of care taken in positioning the aforementioned markings in a static condition, additional error is introduced once the crankshaft begins to rotate. Specifically, assembly clearances in combination with component inertia, stretch and compression increase the difficulty of determining piston TDC relative to crankshaft angle.
Another system incorporates the use of a pressure transducer within the cylinder to determine the maximum cylinder pressure generated while the valves are closed. While this data collection method may be beneficially employed to provide a real time trace of cylinder pressure per crankshaft angle of rotation, difficulty arises when attempting to accurately determine the piston TDC. For example, one current method of determining the piston top dead center location relies on the shape of the log pressure versus log crankshaft angle curve noting that the trace forms a knife edge as the combustion stroke reverses to an expansion stroke. The technique involves manipulating the data collected relative to a reference crankshaft angle and offsetting the data set a number of degrees to achieve the knife edge plot shape. Once the knife edge is graphically determined, TDC is defined relative to the reference crankshaft angle. While this method has been useful for obtaining a rough estimate of the actual TDC position, the method lacks the ability to discern small changes in crank angle such as within a range of 2-3 degrees.
Unfortunately, errors approaching 0.5 degrees in TDC location may significantly affect internal combustion engine performance. Specifically, diesel engine performance is effected by errors in crankshaft angle as small as 0.1 degrees from top dead center. Therefore, a need exists for a simple, real time analysis technique capable of accurately determining the top dead center location in an internal combustion engine.
Accordingly, it is an object of the present invention to define an algorithm for determining the top dead center location of a piston in an internal combustion engine using cylinder pressure measurements.
It is another object of the present invention to provide a graphical method of accurately determining the top dead center location of a piston in an internal combustion engine cylinder piston.
The present invention includes a method of determining a top dead center location of a piston in an internal combustion engine including the steps of determining a pressure versus crankshaft angle relation during a compression and an expansion portion of engine operation, defining a pressure ratio relation as the pressure at x number of degrees before a reference crankshaft angle divided by the pressure at x number of degrees after the reference crankshaft angle and shifting the pressure ratio relation a corrective number of degrees to obtain a model shape. Accordingly, the top dead center position is defined relative to the reference crankshaft angle.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from a reading of the subsequent description of the preferred embodiment and the appended claims, taken in conjunction with the accompanying drawings.